[jjn2009]

>how would you, looking at the photo, know that this is multicultural?

You could take it for face value and just assume they were talking about the food. It's also possible the "white people" are from a variety of the many diverse european cultures. Those both by definition are multicultural.

>Multicultural here is misused as a thinly veiled term for 'there's one not-white person in this photo'

No its not, you are just focusing on the color of their skin and projecting a world view that is not built upon principle but instead identity politics. Try and change out "white people" in your argument for any large group of people who are of other skin colors and this becomes immediately apparent.